Gas-generating pyrotechnic solid objects

ABSTRACT

Gas-generating pyrotechnic solid objects, the composition of which, free of binder and of explosive ingredient, expressed as weight percentages, contains from 35 to 50%, advantageously from 40 to 50%, of guanidine nitrate, from 35 to 50% of basic copper nitrate, from 0.5 to 6% of at least one compound chosen from alumina and inorganic titanates, the melting point of which is above 2100 K, and from 5 to 18% of at least one inorganic oxalate, chosen from sodium oxalate, tin oxalate, strontium oxalate, iron oxalate, copper oxalate and mixtures thereof.

The present invention relates to novel gas-generating pyrotechnic solid objects. Said novel objects are particularly advantageous with regard to their combustion temperature (low), their generation of combustion residues (in a small amount, in the form of agglomerates) and of their production (with easy dry-route implementation). They are suitable for use in gas generators having optimized architecture. This optimization is specified below.

Said gas-generating pyrotechnic solid objects are particularly suitable for use in motor vehicle occupant protection systems, more especially for the inflation of frontal airbags.

The technical field relating to motor vehicle occupant protection has experienced very considerable growth over the last 20 years. Vehicles integrate within the passenger compartment thereof several safety systems of airbag type. Among the airbag-type safety systems, frontal (driver or passenger) airbags and side (curtain, thorax-protection) airbags are distinguished. Frontal airbags differ from side airbags essentially by the time required for the deployment and positioning of the airbag. Typically, this time is longer for a frontal airbag (of the order of 40-50 ms, versus 10-20 ms for a side airbag).

Frontal airbags require for the most part gas generators that are said to be entirely pyrotechnic, which include at least one pyrotechnic charge, consisting of at least one pyrotechnic solid object. This type of design in turn necessitates that said at least one pyrotechnic solid object simultaneously satisfies numerous requirements (relating to its gas yield, to its inflation rate per unit area, to its ignitability, to its combustion temperature and rate, to its pressure exponent, to the non-toxicity of the gases generated by its combustion, to the amount of solid particles generated by its combustion and to the pyrotechnic safety during the obtaining and use thereof).

Various types of pyrotechnic compositions, for gas-generating pyrotechnic solid objects suitable particularly for use in motor vehicle occupant protection systems, have already been proposed to date.

U.S. Pat. No. 6,361,630 describes, in bases of guanidine nitrate type (GN, 15 to 35% by weight, as reducing charge)+strontium nitrate (Sr(NO₃)₂, 30 to 50% by weight, as oxidizing charge), the use, in a relatively large amount (15 to 25% by weight) of an endothermic agent (cooling agent), chosen from alkali metal or alkaline earth metal formates, alkali metal or alkaline earth metal oxalates, and mixtures thereof. This document in fact only illustrates the use of calcium formate in a base which, besides said guanidine nitrate (GN) and strontium nitrate (Sr(NO₃)₂), contains an explosive ingredient (of HMX type) in order to increase the rate of combustion and a binder (of polyethylene/butylene-polystyrene block copolymer type). The objects described are obtained by a dry route.

U.S. Pat. No. 6,602,365 describes the obtention, (at least in part) by a wet route, of gas-generating pyrotechnic solid objects having a composition that contains:

-   -   a complex of guanylurea nitrate with a metal (Cu, Zn, Mn, for         example), such as that of formula CuGuN,     -   an oxidizer (such as basic copper nitrate (BCN)), and     -   another reducing agent (such as guanidine nitrate (GN)).

Said objects have a low combustion temperature, owing to the presence of said complex of low enthalpy of formation in their composition.

Currently, for frontal airbags, the pyrotechnic solid objects that appear to offer the best compromise with reference to the numerous requirements to be satisfied (see above) contain, in their composition, as main ingredients, guanidine nitrate (GN; as reducing charge) and basic copper nitrate (BCN; as oxidizing charge). Their composition (of GN/BCN type, therefore) is furthermore capable of containing at least one additive, which acts on the agglomeration of the combustion residues and/or, advantageously and, on the combustion rate.

The applicant has more particularly described, in patent application WO 2012/153062, objects of this (GN/BCN) type, containing, in their composition, at least one inorganic titanate, the melting point of which is above 2100 K. Said at least one inorganic titanate, present at a low weight content (≤5%), carries out a dual function:

-   -   it acts as a “slagging” agent or agglomerating agent (said at         least one inorganic titanate is a refractory compound, the         melting point of which (see above) is significantly higher than         the combustion temperatures of the (GN/BCN) bases in which it is         present. Thus, it retains its pulverulent solid physical state         (it obviously participates in this form) at the combustion         temperature, which is a necessary characteristic for obtaining         an effect of agglomerating the liquid copper residues (for an         increase in the viscosity of the condensed phase consisting of         liquid copper). The filterability of the combustion residues         being thus facilitated, it is possible to reduce the         gas-generating filtration systems); and,     -   it acts positively on the combustion rate (the objects         containing said at least one inorganic titanate in their         composition simultaneously have a high combustion rate (≥20 mm/s         at 20 MPa) and a moderate combustion temperature (<2200 K), with         a low pressure exponent and nonzero and self-sustaining         combustion at atmospheric pressure).

The applicant has in fact considered the technical problem of the space requirement and the weight (and therefore the cost) of gas generators operating with a gas-generating pyrotechnic solid charge. It desired to optimize the architecture of said generators by minimizing the volume and the weight of the devices required, within the structure of said generators, for the filtration and cooling of the combustion gases generated. It may be indicated, as an illustration, that the weight of the filtration device used, for a gas-generating composition, the combustion temperature of which is 1900 K, is generally equivalent to the weight of the gas-generating charge. For all practical purposes, it is also possible to specify that the filtration device constitutes per se a cooling device (which combines its cooling effect with that of the cooling device).

From this point of view of optimizing the architecture of said generators, the applicant proposes novel gas-generating pyrotechnic solid objects, having a composition of GN/BCN type, which may be obtained by a dry route, without the presence of binder in their composition, and the combustion temperature of which (below 1800 K) is lower than that (below 2200 K) of the objects described in patent application WO 2012/153062. Said combustion temperature is lowered by the presence, within the composition of the objects, of at least one specific cooling agent (see below); said presence of said at least one specific cooling agent, in a relatively limited amount (≤18% by weight, in particular ≤15% by weight, and even ≤13% by weight), within a specific base (see its composition specified below), being effective (with regard therefore to the lowering of the combustion temperature) while inducing only limited effects on the other parameters such as the combustion rate (the presence of an explosive ingredient not being required), the gas yield and the temperature stability of the objects involved.

According to its first subject, the present invention therefore relates to gas-generating pyrotechnic solid objects. Characteristically, the composition of said objects, expressed as weight percentages, contains:

-   -   from 35 to 50%, advantageously from 40 to 50%, of guanidine         nitrate (GN),     -   from 35 to 50% of basic copper nitrate (BCN),     -   from 0.5 to 6% of at least one compound chosen from alumina         (Al₂O₃), inorganic titanates, the melting point of which is         above 2100 K, and mixtures thereof, and     -   from 5 to 18% of at least one inorganic oxalate, chosen from         sodium oxalate (Na₂C₂O₄), tin oxalate (SnC₂O₄), strontium         oxalate (SrC₂O₄), iron oxalate (FeC₂O₄), copper oxalate (CuC₂O₄)         and mixtures thereof;         said composition being, furthermore, free of binder and of         explosive ingredient.

Said composition is therefore of GN (reducing charge)/BCN (oxidizing charge) type.

Guanidine nitrate (GN) was used as reducing agent, inter alia, for its ability to generate a lot of gas, for its rheoplastic behavior suitable for the implementation of the (direct) pelletizing phase or of the compacting and pelletizing phases of a dry-route process (its presence enables in particular a good densification of the starting pulverulent pyrotechnic composition while limiting the compressive stress to be applied: see below), and for pyrotechnic safety reasons. The composition of the pyrotechnic objects of the invention contains from 35 to 50% by weight of guanidine nitrate (GN), advantageously from 40 to 50% by weight of guanidine nitrate (GN).

Basic copper nitrate (BCN) was used as oxidizing agent, this being, very particularly, for its impact on the combustion rate, for its ductibility and its “slagging” effect (presence of copper). Said basic copper nitrate is present in an amount from 35 to 50% by weight, generally in a proportion from 35 to 45% by weight.

Said composition therefore contains, in a base of GN/BCN type (as specified above, with reference in particular to the desired oxygen balance value, close to −3%), a small amount (from 0.5 to 6%, often from 1 to 6%, advantageously from 3 to 5%, by weight) of at least one compound chosen from alumina (Al₂O₃), inorganic titanates, the melting point of which is above 2100 K, and mixtures thereof. The presence of said at least one compound is opportune with regard to the agglomeration of the combustion residues (for all practical purposes, it may be noted here that alumina has a greater “slagging” power than that of the titanates) and the combustion rate. Said composition advantageously contains alumina (Al₂O₃) or at least one titanate as specified above. Very advantageously it contains alumina (Al₂O₃) or such a titanate.

Said composition of the objects of the invention contains, in a base of GN/BCN type as specified above, in addition to said at least one inorganic titanate and/or alumina, a relatively limited amount (from 5 to 18%, in particular from 5 to 15%, advantageously from 5 to 13%, very advantageously from 7 to 13%, by weight) of at least one specific cooling agent, i.e. of at least one inorganic oxalate chosen from sodium oxalate (Na₂C₂O₄), tin oxalate (SnC₂O₄), strontium oxalate (SrC₂O₄), iron oxalate (FeC₂O₄), copper oxalate (CuC₂O₄) and mixtures thereof. As indicated above, the presence, within the specified GN/BCN base (containing said at least one inorganic titanate and/or alumina, of at least one such oxalate, in said relatively limited amount indicated, has proved opportune, with reference to the lowering of the combustion temperature of the objects, without inducing significant effects on the other parameters such as the combustion rate (the presence of an explosive ingredient is not required), the gas yield (oxalates were preferred to formates) and the stability over time (the selected oxalates not being very hygroscopic) and temperature stability (the melting point and/or decomposition temperature of the oxalates used is not below 200° C. (a person skilled in the art has understood that said oxalates used are in anhydrous form)) of said objects. The selected oxalates are furthermore non-toxic and of reasonable cost.

Said composition of the objects of the invention does not contain a binder. Specifically, the rheoplastic behavior of guanidine nitrate (GN), being incorporated in a significant amount in said composition, makes the presence of any binder superfluous (in particular for obtaining, via the dry route, shaped pyrotechnic objects, granules, pellets and compressed monolithic blocks (see below)). A person skilled in the art understands the advantage of being able to obtain, via the dry route, the objects of the invention, and this being without the participation of a binder (which would have a significant impact on the oxygen balance of the composition); the absence of any binder furthermore being particularly opportune with reference to the targeted objective of low combustion temperature and of high gas yield of said objects.

Said composition of the objects of the invention does not contain an explosive ingredient. It thus contains neither nitroguanidine, nor hexogen (RDX), nor octogen (HMX), etc. An explosive ingredient is understood, currently and conventionally, to mean the ingredients classified under risk division 1.1 according to the standard NF T 70-502 (see also UN Recommendations on the Transport of Dangerous Goods—Manual of Tests and Criteria, Fourth revised edition, ST/SG/AC.10/11/Rev.4, ISBN 92-1-239083-8155N 1014-7179 and STANAG 4488). For all practical purposes, it is recalled that guanidine nitrate (GN) is not an ingredient that is classified under this risk division. The absence of any explosive ingredient within the composition of the objects of the invention is particularly opportune with reference to the safety and to the combustion temperature of said objects. It is recalled incidentally that a low combustion temperature is desired.

The objects of the invention, having the composition as specified above, have therefore proved particularly advantageous with regard to:

1) their combustion temperature (low: below 1800 K; which low combustion temperature remains associated with a combustion rate of more than 15 mm/s at 20 MPa (with reference to said combustion rate, it is already possible to indicate here that it is opportunely increased by using the ingredients in appropriate fine particle sizes (see below)), 2) their generation of combustion residues (in a small amount, in the form of agglomerates), and 3) their obtention (with easy dry-route implementation).

The at least one inorganic oxalate present in the composition of the objects of the invention is advantageously chosen from sodium oxalate (Na₂C₂O₄), strontium oxalate (SrC₂O₄), and copper oxalate (CuC₂O₄).

The at least one inorganic titanate, the melting point of which is above 2100 K, optionally present in the composition of the objects of the invention (it is recalled for all practical purposes that said composition contains from 0.5 to 6%, often from 1 to 6%, advantageously from 3 to 5%, of at least one compound chosen from alumina (Al₂O₃), inorganic titanates, the melting point of which is above 2100 K, and mixtures thereof), is advantageously chosen from metal titanates, alkaline-earth metal titanates and mixtures thereof. Very advantageously it consists of a metal titanate or an alkaline-earth metal titanate. Preferably, the composition of the objects of the invention, which contains at least one titanate as specified, contains strontium titanate (SrTiO₃) and/or calcium titanate (CaTiO₃) and/or aluminum titanate (Al₂TiO₅). Particularly preferably, it contains strontium titanate (SrTiO₃), calcium titanate (CaTiO₃) or aluminum titanate (Al₂TiO₅). These titanates respectively have melting points of 2353 K, 2248 K and 2133 K, i.e. melting point significantly above the combustion temperature of the GN/BCN base (the combustion temperature of any GN/BCN base specifically always being below 1950 K), which moreover contains the at least one inorganic oxalate.

Within the context of its first subject, the present invention relates to the subfamily of gas-generating pyrotechnic solid objects, the composition of which, expressed as weight percentages, contains:

-   -   from 35 to 50%, advantageously from 40 to 50%, of guanidine         nitrate (GN),     -   from 35 to 45% of basic copper nitrate (BCN),     -   from 1 to 6%, advantageously from 3 to 5%, of at least one         compound chosen from alumina (Al₂O₃), inorganic titanates, the         melting point of which is above 2100 K, and mixtures thereof,         and     -   from 5 to 15%, advantageously from 5 to 13%, very advantageously         from 7 to 13%, of at least one inorganic oxalate chosen from         sodium oxalate (Na₂C₂O₄), tin oxalate (SnC₂O₄) and mixtures         thereof; said composition being, furthermore, free of binder and         of explosive ingredient.

In the composition of the objects of this subfamily, the at least one inorganic oxalate advantageously consists of sodium oxalate.

With regard to the objects of said subfamily, what was stated above and which very obviously applies thereto can be repeated.

The ingredients of the four types above (guanidine nitrate (GN), basic copper nitrate (BCN), alumina and/or inorganic titanate(s), the melting point of which is above 2100 K, and inorganic oxalate(s), as specified) (constituent ingredients of the objects of the invention in general and of the subfamily above in particular) generally represent at least 98% by weight of the composition of the pyrotechnic objects of the invention. The ingredients of the four types above may entirely represent at least at least 99.5% by weight, or even 100% by weight of the total weight of the objects of the invention. The optional presence of at least one “other” additive (alumina and/or the at least one inorganic titanate and also the at least one inorganic oxalate being entirely able to be likened to additives), chosen, for example, from manufacturing aids (calcium stearate, graphite, silica in particular), is expressly anticipated, at a content of less than or equal to 2% by weight. It is understood, in view of the above remarks, that such an at least one “other” additive could not in any way consist of a binder or of an explosive ingredient.

The (main) constituent ingredients of the objects of the invention—guanidine nitrate+basic copper nitrate+alumina and/or at least one inorganic titanate as specified+at least one inorganic oxalate as specified—are known products. They are in the form of powders, the particle size distribution of which is narrow (around their median diameter (d₅₀)). Throughout the present text (including in the examples), the median diameters indicated are median diameters by volume.

Said (main) constituent ingredients of the objects of the invention advantageously have, very particularly with reference to obtaining said objects via the dry route and to the combustion rate of said objects, a fine, or even very fine particle size. They generally have median diameter (d₅₀) values of less than or equal to 20 μm.

For a perfect mixing of the powders of the GN/BCN base (main constituent ingredients of the objects of the invention, conventionally participating in the pulverulent state), it is recommended that the median diameter of one of said guanidine nitrate (GN) and basic copper nitrate (BCN) be substantially higher than the median diameter of the other of said guanidine nitrate (GN) and basic copper nitrate (BCN), said substantially (significantly) higher median diameter generally remaining less than or equal to 20 μm (see above). The expression “substantially higher” is understood to mean “at least 1.8 times higher”, advantageously “at least double”, very advantageously “at least 5 times higher”, or even “at least 10 times higher”. Very advantageous results have in particular been obtained with GN powders having a median diameter of 12 μm and BCN powders having a median diameter of less than 6 μm. According to one advantageous variant, the median diameter of one of said guanidine nitrate and basic copper nitrate, for example that of said basic copper nitrate, is less than or equal to 1 μm whereas the median diameter of the other of said guanidine nitrate and basic copper nitrate, for example therefore that of said guanidine nitrate, is at least 5 μm, advantageously at least 10 μm (while generally remaining less than or equal to 20 μm (see above)). Very advantageous results have in particular been obtained with GN powders having a median diameter between 10 and 14 μm and BCN powders having a median diameter of 1 μm. The participation of a very fine powder (d₅₀≤1 μm) and of a “substantially coarser” powder is recommended very particularly with a view to the creation of a perfect mixture and the obtention of a high combustion rate. This (positive) effect of the particle size on the combustion rate is opportunely taken advantage of in order to compensate for the limited effect of the presence of the oxalate on said combustion rate.

When alumina (it too participating in the pulverulent state) is present, it is advantageously present at a fine, or even very fine particle size; it then has a high, or even very high specific surface area. Mention has been made, for all the constituent ingredients, of median diameter values generally less than or equal to 20 μm. For alumina, mention may be made of median diameter values generally less than or equal to 10 μm, advantageously less than or equal to 5 μm, very advantageously less than or equal to 1 μm, or even as low as 100 nm, and even 10 nm.

When at least one inorganic titanate (it too participating in the pulverulent state) is present, it is also advantageously present at the smallest possible particle size. Thus, advantageously, said at least one inorganic titanate present is in fine pulverulent form, of micrometer dimension, or even of nanometer dimension, i.e. with a median diameter (d₅₀) of less than 6 μm, or even less than 1 μm (generally within the context of this advantageous variant: 0.5 μm≤d₅₀≤6 μm). Said at least one inorganic titanate present advantageously has a specific surface area of greater than 1 m²/g (very advantageously greater than 5 m²/g or more).

As regards the particle size of the at least one inorganic oxalate, it is itself also opportunely as fine as possible. However, good results have been obtained with commercial products “of coarse particle size”, in particular with sodium oxalate having a median diameter of greater than 40 μm (of 60 μm in particular). No doubt the good results obtained would be even better with (at least) one finer oxalate of the invention (in particular having a median diameter of less than 20 μm (see above)).

The objects of the invention are in particular capable of existing in the form of shaped pyrotechnic objects, of granules, of pellets or of monolithic (compressed) blocks (see below).

The manufacture of the pyrotechnic solid objects of the invention will now be dealt with. The manufacturing processes involved are, by analogy, dry route or wet route processes, advantageously dry route processes.

Dry Route

The pyrotechnic solid objects of the invention may be manufactured (by the dry route) by simple pelletizing (compression) of the mixtures of powders obtained by mixing the constituent ingredients thereof (it is understood that said ingredients are used in the pulverulent state, with a relatively fine particle size, opportunely as fine as possible (see above), and that they are essentially, or even exclusively GN, BCN, alumina and/or inorganic titanate(s) as specified, and oxalate(s) as specified).

The pyrotechnic objects of the invention may also be manufactured (by the dry route) according to a process capable of comprising up to four main steps. Such a process is familiar to person skilled in the art. It has in particular been described in patent application WO 2006/134311.

The alumina or (and) the at least one inorganic titanate (the melting point of which is above 2100 K) and the at least one inorganic oxalate (chosen from sodium oxalate, tin oxalate, strontium oxalate, iron oxalate, copper oxalate and mixtures thereof) advantageously participate with the other constituent ingredients, GN+BCN mainly, or even GN+BCN exclusively, at the start of the manufacturing process. It is however possible that said alumina or (and) said at least one inorganic titanate (the melting point of which is above 2100 K) or (and) said at least one inorganic oxalate (chosen from sodium oxalate, tin oxalate, strontium oxalate, iron oxalate, copper oxalate and mixtures thereof), very particularly said at least one inorganic oxalate, is (are) added, further downstream, in the process for manufacturing the objects of the invention. It is understood that several alternatives exist. Incidentally it may be noted that it is not ruled out to involve one and/or the other of said alumina, at least one inorganic titanate and at least one inorganic oxalate in several goes in the course of said process.

The preferred dry route process for manufacturing (preparing) the pyrotechnic objects of the invention includes a step of dry compacting a mixture of the powdered constituent ingredients of said objects (with the exception, optionally, of said at least one inorganic oxalate which may be added later). The dry compacting is generally carried out in a manner known per se, in a roll compactor, at a compacting pressure (p) of between 10⁸ and 6.10⁸ Pa (10⁸ Pa≤p≤6.10⁸ Pa). It can be carried out according to different variants (with a characteristic step of “simple” compacting followed by at least one additional step or with a characteristic step of compacting coupled to a shaping step).

Thus, the pyrotechnic solid objects of the invention are capable of existing in various forms (in particular during the manufacturing process resulting in the final objects):

-   -   at the end of dry compacting coupled with shaping (by using at         least one compacting roll, the external surface of which has         cavities), sheets are obtained with relief patterns, which can         be broken to directly obtain shaped pyrotechnic objects;     -   at the end of dry compacting (“simple” compacting which         generates a flat sheet) followed by granulation, granules are         obtained;     -   at the end of dry compacting (“simple” compacting which         generates a flat sheet) followed by granulation then pelletizing         (dry compression), pellets or (compressed) monolithic blocks are         obtained.

The objects of the invention—shaped pyrotechnic objects, granules, pellets and monolithic blocks—obtained at the end of one or other of the steps specified above are particularly preferred, it being understood that pellets can also be obtained by direct pelletizing (see above).

Wet Route

The pyrotechnic solid objects of the invention may also be obtained by a wet route process. Such a process includes 1) a step of placing all the or (generally rather) some of the main ingredients in aqueous solution (said step of placing in aqueous solution generally comprises dissolving at least one of the main ingredients (and more particularly dissolving guanidine nitrate (GN))), 2) obtaining a powder by spray drying, 3) adding the ingredient(s) that were not placed in solution to the powder obtained, then 4) shaping (in the form of objects) the pulverulent mixture obtained by the standard dry route processes.

The objects of the invention advantageously exist in the form of granules, pellets or monolithic blocks.

In a manner which is in no way limiting, it may be indicated here:

-   -   that the granules of the invention generally have a median         diameter (d₅₀) of between 200 and 1000 μm (and also a bulk         density of between 0.8 and 1.2 g/cm³); and     -   that the pellets of the invention generally have a thickness of         between 1 and 6 mm for a diameter of 3 to 15 mm.

Said granules and pellets are perfectly suitable for the main application targeted (that of frontal airbags, in the field of motor vehicle safety). The shaped pyrotechnic objects and monolithic blocks are intended for other uses.

According to another of its subjects, the present invention relates to a pulverulent composition (mixture of powders), which is a precursor of an object of the invention, the composition of which therefore corresponds to that of an object of the invention (see above).

According to another of its subjects, the present invention relates to gas generators containing a gas-generating pyrotechnic solid charge; said charge containing at least one pyrotechnic solid object of the invention. Said generators, loaded in particular with pellets of the invention, are perfectly suitable for airbags, in particular frontal airbags (see above).

It is now proposed to illustrate the invention in a manner which is in no way limiting.

Pellets (pellets with a diameter of 11 mm and a thickness of 3 mm) were produced from the ingredients below:

-   -   GN (sold by Alzchem AG (DE), of 10-14 μm grade), (d₅₀≈12 μm),     -   BCN (sold by Shepherd Chemical Company (US), of 4-6 μm grade),         (d₅₀≈5 μm),     -   BCN (d₅₀≈1 μm),     -   SrTiO₃ (sold by Thermograde Process Technology Ltd (US), of 4-6         μm grade), (d₅₀≈5.5 μm),     -   Al₂O₃ (sold by Evonik Industries AG (DE), of 10-100 nm grade),         (specific surface area: 100 m²/g),     -   Na₂C₂O₄ (sold by Alfa Aesar (US), of 40-70 μm grade, (d₅₀≈55         μm),     -   SrC₂O₄ (sold by Isaltis (FR), (d₅₀≈4 μm)     -   CuC₂O₄ sold by Bernardy (FR), (d₅₀≈5 μm), via a direct         compression dry route process (=simple pelletizing carried out         with a pressure of 45×10⁶ Pa (450 bar)).

Table 1 below presents five examples (Ex.1, Ex.2, Ex.3, Ex.4 and Ex.5) of the composition of objects (pellets) of the present invention, and also the characteristics (calculated or measured performances) of said objects (pellets) compared to those of an object (pellet) of the prior art (Ref.1, according to patent application WO 2012/153062); said objects (pellets) of the invention and of the prior art having been manufactured, from the ingredients above, as indicated above.

Table 1 below also presents two other examples (Ex. A and Ref. 2) of the composition of pellets and also the performances of said pellets (obtained in a similar manner). These examples highlight the advantage of using BCN having a very fine particle size (with GN having a substantially higher particle size).

The objects (pellets) were evaluated by means of thermodynamic calculations and from physical measurements performed therefore on the pellets. The combustion rates and pressure exponents of said pellets were obtained following several firings in a manometric chamber (volume 40 cm³). The values indicated are therefore average values.

The reference prior art pellets (Ref.1) contained, in their composition, guanidine nitrate (GN), basic copper nitrate (BCN) and also strontium titanate (SrTiO₃), in the weight percentages indicated.

The pellets of examples 1 to 5 (Ex.1, Ex.2, Ex.3, Ex.4 and Ex.5) contained, in their composition, in addition to the three constituents guanidine nitrate (GN), basic copper nitrate (BCN) and strontium titanate (SrTiO₃) for examples 1 and 2, alumina for examples 3, 4 and 5, an agent for lowering the combustion temperature according to the present invention: sodium oxalate (Na₂C₂O₄) for examples 1 to 3, strontium oxalate (SrC₂O₄) for example 4 and copper oxalate (CuC₂O₄) for example 5. The four constituents were present in the compositions in the weight percentages indicated.

The characteristics (performances) of the compositions (objects) of examples 1 and 2 showed that the addition, in a variable amount (weight content of 7.5 and 10% respectively), of sodium oxalate (Na₂C₂O₄), in a composition of the type of that of the reference 1 (Ref. 1), resulted in a significant lowering of the combustion temperature (by −144° C. and −190° C. respectively). The value of the pressure exponent remained acceptable for the targeted application (frontal airbags).

The characteristics (performances) of the composition (object) of example 3 showed that the presence of alumina contributed to obtaining good performances (lowering of the combustion temperature (by −155° C. compared to the Ref.1 example) with an increase in the value of the combustion rate at 20 MPa compared to example 2, and a parallel increase in the agglomeration quality of the combustion residues compared to example 2).

The characteristics (performances) of the composition (object) of example 4 showed that the combined addition of alumina and strontium oxalate (SrC₂O₄) in a composition of the type of that of the reference 1 (Ref. 1), resulted in a significant lowering of the combustion temperature (of −156° C.). The value of the pressure exponent remained acceptable for the targeted application (frontal airbags). The very good agglomeration quality of the combustion residues must be highlighted.

The characteristics (performances) indicated for the composition (object) of example 5 showed that the combined addition of alumina and copper oxalate (CuC₂O₄) in a composition of the type of that of the reference 1 (Ref. 1), resulted in a significant lowering of the combustion temperature (of −146° C.).

Example A, to be considered therefore in parallel with the reference example 2 (Ref. 2), illustrates the participation of basic copper nitrate (BCN) of fine grade (d₅₀=1 μm) in a composition of GN type (of substantially higher particle size)+BCN+Al₂O₃ (2.7%), in the absence of oxalate (more particularly sodium oxalate, strontium oxalate and copper oxalate). The impact on the combustion rate is significant: increase of more than 20% (in comparison with the combustion rate of the pellets of the reference example 2 (Ref. 2)). Thus it is confirmed that it is possible, within the context of the present invention, to lower the combustion temperature while optimizing the combustion rate via the small particle size of the BCN present (in fact by the small particle size of one of the constituents GN or BCN, the other of said constituents then having a substantially larger particle size).

TABLE 1 Examples Ref. 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ref. 2 Ex. A Ingredients Guanidine nitrate (GN) % 52 45.7 44 46.3 43.9 42.8 52.7 52.7 Basic copper nitrate (BCN) % 44 42.8 42 42.7 41.9 39.5 44.6 — Basic copper nitrate % — — — — — — — 44.6 “fine grade” (BCN) Strontium titanate (SrTiO₃) % 4 4 4 — — — — — Alumina (Al₂O₃) % — — — 1 2.7 2.7 2.7 2.7 Sodium oxalate (Na₂C₂O₄) % — 7.5 10 10 — — — — Strontium oxalate (SrC₂O₄) % — — — — 11.5 — — — Copper oxalate (CuC₂O₄) % — — — — — 15 — — Characteristics Oxygen balance % −3.3 −2.7 −2.8 −3.4 −2.7 −2.7 −3.4 −3.4 Combustion temperature K. 1889 1745 1699 1734 1733 1743 1899 1899 at 20 MPa Density g/cm³ 2.01 1.937 1.943 1.934 2.074 2.143 1.881 1.881 Gas yield at 1 bar - 1000K mole/kg 29.1 26.8 26.1 27.2 26.2 26.2 29.6 29.6 Residue content at 1 bar - 1000K % 27.6 31.6 33.2 31.1 32.1 30.9 26.3 26.3 Combustion rate at 20 MPa mm/s 18.8 15.8 15.4 19.6 15.1 19.5 23.4 Pressure exponent 0.13 0.24 0.34 0.28 0.27 0.09 0.19 (16 to 29 MPa range) Agglomerated appearance of the Good Average Average Good Very Very Very combustion residues in the form of good good good a backbone of the pyrotechnic block 

1. A gas-generating pyrotechnic solid object, the composition of which, expressed as weight percentages, contains: from 35 to 50% of guanidine nitrate, from 35 to 50% of basic copper nitrate, from 0.5 to 6% of at least one compound selected from the group consisting of alumina and inorganic titanates, the melting point of which is above 2100 K, and from 5 to 18% of at least one inorganic oxalate, selected from the group consisting of sodium oxalate, tin oxalate, strontium oxalate, iron oxalate, copper oxalate and mixtures thereof; said composition being free of binder and of explosive ingredient.
 2. The object as claimed in claim 1, wherein said at least one inorganic oxalate is selected from the group consisting of sodium oxalate, strontium oxalate and copper oxalate.
 3. The object as claimed in claim 1, the composition of which, expressed as weight percentages, contains: from 35 to 50% of guanidine nitrate, from 35 to 45% of basic copper nitrate, from 1 to 6%, advantageously 3 to 5%, of at least one compound selected from the group consisting of alumina and inorganic titanates, the melting point of which is above 2100 K, and from 5 to 15% of at least one inorganic oxalate selected from the group consisting of sodium oxalate, tin oxalate and mixtures thereof.
 4. The object as claimed in claim 1, wherein said at least one inorganic titanate chosen is selected from the group consisting of strontium titanate, calcium titanate, aluminum titanate and mixtures thereof.
 5. The object as claimed in claim 1, the composition of which consists, for at least 98% by weight of said guanidine nitrate, basic copper nitrate, alumina and/or inorganic titanate(s), and inorganic oxalate(s).
 6. The object as claimed in claim 1, wherein said guanidine nitrate, basic copper nitrate, alumina and/or inorganic titanate(s), and inorganic oxalate(s) have median diameters of less than or equal to 20 μm.
 7. The object as claimed in claim 1, wherein the median diameter of one of said guanidine nitrate and basic copper nitrate is substantially higher than the median diameter of the other of said guanidine nitrate and basic copper nitrate.
 8. The object as claimed in claim 1, wherein the median diameter of one of said guanidine nitrate and basic copper nitrate is less than or equal to 1 μm whereas the median diameter of the other of said guanidine nitrate and basic copper nitrate is at least 5 μm.
 9. The object as claimed in claim 1, the composition of which contains alumina, wherein the median diameter of said alumina is less than or equal to 5 μm.
 10. The object as claimed in claim 1, the composition of which contains at least one inorganic titanate, the median diameter of which is less than 6 μm.
 11. A process for preparing the object as claimed in claim 1, which is a dry-route process, said process comprising a step of compacting a pulverulent mixture containing said guanidine nitrate, basic copper nitrate, alumina and/or inorganic titanate(s), and inorganic oxalate(s) in a powdered form.
 12. The object as claimed in claim 1, which is in a form selected from the group consisting of granules, pellets and monolithic blocks.
 13. A pulverulent composition, which is a precursor of an object as claimed in claim
 1. 14. A gas generator, containing a gas-generating pyrotechnic solid charge, wherein said charge contains at least one object as claimed in claim
 1. 15. The object as claimed in claim 1, the composition of which, expressed as weight percentages, contains: from 40 to 50% of guanidine nitrate, from 35 to 45% of basic copper nitrate, from 3 to 5%, of at least one compound selected from the group consisting of alumina and inorganic titanates, the melting point of which is above 2100 K, and from 5 to 13% of at least one inorganic oxalate selected from the group consisting of sodium oxalate, tin oxalate and mixtures thereof.
 16. The object as claimed in claim 1, the composition of which consists, for at least 99.5% by weight, of said guanidine nitrate, basic copper nitrate, alumina and/or inorganic titanate(s), and inorganic oxalate(s).
 17. The object as claimed in claim 1, the composition of which consists, for 100% by weight, of said guanidine nitrate, basic copper nitrate, alumina and/or inorganic titanate(s), and inorganic oxalate(s).
 18. The object as claimed in claim 1, wherein said guanidine nitrate, basic copper nitrate, alumina and/or inorganic titanate(s), and said inorganic oxalate(s) have median diameters of less than or equal to 20 μm; wherein the median diameter of one of said guanidine nitrate and basic copper nitrate is substantially higher than the median diameter of the other of said guanidine nitrate and basic copper nitrate, and wherein the median diameter of said alumina is less than or equal to 5 μm; and wherein the median diameter of said at least one inorganic titanate is less than 6 μm.
 19. The object as claimed in claim 1, wherein said guanidine nitrate, basic copper nitrate, alumina and/or inorganic titanate(s), and said inorganic oxalate(s) have median diameters of less than or equal to 20 μm; and wherein the median diameter of one of said guanidine nitrate and basic copper nitrate is less than or equal to 1 μm whereas the median diameter of the other of said guanidine nitrate and basic copper nitrate is at least 5 μm.
 20. A process for preparing the object as claimed in claim 1, which is a dry-route process, said process comprising a step of compacting a pulverulent mixture containing said guanidine nitrate, basic copper nitrate, alumina and/or inorganic titanate(s), and inorganic oxalate(s) in a powdered form; said step of compacting a pulverulent mixture being followed by a step selected from the group consisting of a step of granulation, a step of palletization, and a step of granulation followed by a step of palletization. 